A. Prior Art
The use of phosphating compositions for inhibiting corrosion on metal substrates and improving the adhesion of superimposed organic coatings, such as paint, is an old and crowded art.
Phosphating compositions, typically applied by immersion of the product to be coated in a bath solution or by spraying, commonly have been used in the form of acidic, aqueous solutions typically containing phosphate ions, an oxidizing agent, and divalent, layer-forming metal cations. The layer-forming ions typically included zinc used alone or in combination with cations of barium, calcium, cobalt, manganese, magnesium, nickel, lithium, and other metals. The phosphate ions are commonly introduced by use of phosphoric acid. The oxidizing agents are commonly inorganic compounds, often consisting of the salt of one of the above-mentioned metals or of sodium or ammonia. Much of this art, as used today, has changed little over the years.
B. Specific Prior Art
Prior art utilizing both zinc cations and other divalent metal cations in the bath is illustrated by the following patents.
U.S. Pat. No. 3,810,792 to Ries teaches a process for applying phosphate coatings on steel, iron or zinc from an aqueous, acidic solution containing divalent, layer-forming metal cations, wherein 59 to 100 mole percent are nickel cations and the remaining 0 to 41 mole percent are cations other than nickel cations, e.g., zinc cations. This invention is exemplified in Example 1 with nickel cations comprising 100 mole percent of the divalent, layer-forming metal cations are in Examples 2, 3 and 4 with nickel cations comprising between 68 and 69 mole percent of the divalent, layer-forming cations.
U.S. Pat. No. 3,090,709 to J. A. Hendricks teaches a process for phosphate coating of metals with a view to forming an "amorphous coating." (See lines 29 and 38 of column 1; lines 11 and 14 of column 2; line 25 of column 3; line 32 of column 7, etc . . . ) His coatings are further "characterized by the substantial absence of crystalline structure" (column 1, lines 18-19), and one object is "inhibiting the formation of crystals (column 1, lines 32-33). He seeks a smoother base for paint, thereby enhancing its gloss. The amorphous type coating "is a mixed phosphate composed principally of zinc and one of the metals selected from . . . lithium, beryllium, magnesium, calcium, strontium, cadmium and barium" (column 2, lines 14-17). Hendricks states at line 65 of column 3 that "the chemical analysis of these amorphous type coatings reveals that they are mixed phosphates with the metal of the recited group occurring therein in the ratio of about one-half mole thereof to each mol of zinc." Hendricks further states at line 34 of column 5 that the amount of modifier metal must be adjusted so that lighter metals such as lithium and magnesium nitrates are added in greater quantities than the nitrates of the heavier elements such as barium. The minimum concentration for magnesium nitrate is given in the lower table in column 5 as 25.6 gr./liter or 10 mole percent in a solution containing 3.2 g/l zinc. Special coating conditions for an embodiment using magnesium are set forth in column 12 at lines 58-60, i.e., the reader is instructed to operate the process at room temperature and the immersion time kept below one minute.
U.S. Pat. No. 3,218,200 to J. A. Hendricks issued from a divisional application taken from the application upon which the aforedescribed U.S. Pat. No. 3,090,709 issued. Hence, it is essentially the same disclosure to be found in U.S. Pat. No. 3,090,709. There is an added paragraph in column 3, at lines 47-50, wherein the term "micro-crystalline" is used to characterize the coatings.
U.S. Pat. No. 4,231,812 to Paulus et al teaches a process for coating hot metal strips (above 250.degree. C.) with a phosphate film by quenching the heated strips in a phosphating bath having a temperature of 80.degree. C. or greater and containing one or more phosphates of the type Me (H.sub.2 PO.sub.4).sub.n, wherein Me may be zinc, nickel, manganese, or alkali metal. No specific phosphating bath compositions are disclosed.
U.S. Pat. No. 4,053,328 to Oka et al teaches zinc phosphate solutions containing zinc ions in an amount, preferably, of at least 0.03 percent by weight and nickel ions in an amount of at least 0.01 percent by weight and in the nickel ion to zinc ion ratio of less than 1.89 to 1. This ratio may vary from 1.89:1 to 0.014:1.
U.S. Pat. No. 4,110,128 to Dreulle et al teaches a phosphate solution containing both zinc and nickel. In Table I, the zinc is present in an amount equivalent to 10 to 50 grams of anhydrous zinc chloride and 0.5 to 20 grams of hexahydrated nickel chloride per liter of final solution may be added. In Table II, 20 g. of anhydrous zinc chloride are used with 5 g. of crystallized hexahydrated nickel chloride.
U.S. Pat. No. 4,153,479 to Ayano et al teaches an acidic, oxidant-free zinc phosphate which contains nickel. They disclose in column 2 at line 17 that "The zinc ion should be present in an amount of higher than 0.01% by weight, generally from 0.01 to 0.2% by weight." They disclose in column 2, at line 66, that "the desired nickel ion concentration ranges from 0.01 to 0.2% by weight."
U.S. Pat. No. 3,723,334 to J. I. Maurer teaches a process for decreasing the scale formation in zinc phosphate composition by adding a carbohydrate. Phosphating compositions disclosed include 0.1-50 g/l zinc and may contain 0.001 to 0.4 wt.% nickel. In Example 3, the only example containing both zinc and nickel, the quantity of Zn.sup.++ is given as 0.14 and the quantity of Ni.sup.++ is given as 0.03. No unit of measurement is given for this example. Assuming that the unit either wt.% or g/l, the nickel component comprises less than 50 mol.% of the zinc/nickel component of the bath.
U.S. Pat. No. 2,554,139 to R. F. Drysdale teaches acidic phosphate compositions 2, 3, 4 or more cationic metals and is concerned with reducing the time of coating. In column 3, lines 1-7, they state that optimum results are obtained when zinc, cobalt and nickel are present in concentration of 0.048, 0.017 and 0.074 gm. per liter respectively. The concentrations in the phosphating bath are given as 0.00192, 0.00068 and 0.00296 gram per liter respectively. This is roughly equivalent to an atomic ratio of 3:2:6.
U.S. Pat. No. 4,182,637 to Otrhalek et al teaches a rinse step following a conventional phosphating treatment. A zinc phosphate solution is disclosed in Example 1 containing by weight 0.8% zinc ions, 2.4% phosphate ions, 0.07% nickel ions, 0.6% nitrate ions and 0.3% ferrous ions.
U.S. Pat. No. 4,265,677 to Muller et al relates to a special phosphating solution to be used prior to cathodic electropainting and is concerned with the ratio of zinc to fluoroborate. A zinc/nickel phosphate solution is disclosed in Example 1 which contains 0.69 g/l:Zn and 0.38 g/l:Ni.
U.S. Pat. No. 3,520,737 to Gerassinoff et al teaches a method for applying an aqueous acidic zinc phosphate solution which may contain such as a metallic catalyst nickel, cobalt, or copper in the form of soluble salts in small amounts such as 0.0025% nickel. Somewhat larger amounts of copper and/or nickel and/or cobalt. For heavier coating weights, they advocate use of these metals in amounts exceeding 0.001%. It is also disclosed that when these metals are present in greater amounts of 0.0055%-0.0165% autocatalytic nitrite-formation is promoted.
Oppen et al, U.S. Pat. No. 4,264,378, teaches a process for preparing metal surfaces with a phosphating liquid, containing at least one metal cation of valence two or greater (calcium, magnesium, barium, aluminum, zinc, cadmium, iron, nickel, cobalt and manganese), and contains at least one ion selected from molybdate, tungstate, nandate, niobate, and tantalate ions. In Example 5, Oppen et al discloses a bath containing 6.5 g/l zinc and 5.5 g/l nickel, i.e., the nickel component comprised less than 50 mol.% of the combined zinc/nickel component of the bath. The use of this process for preparing surfaces of iron, zinc or aluminum, or their alloys, is disclosed, but the examples are limited to the coating of aluminum.
C. Increased Need for Greater Corrosion Protection
The need for phosphate coatings of consistently higher quality and much greater corrosion resistance has risen with the increase in the use of road deicing salts. The use of road salt in the snow belt areas of the United States and Canada has increased rapidly from about one million tons per year in the late 1950's to over 10 million tons per year at present.
For many years it was not understood why a scratch in the paint and phosphate film on the exterior of a car body produced corrosion failure over an area much greater than the width of the scratch itself. In a classic study by R. R. Wiggle, A. G. Smith and J. V. Petrocelli, published in The Journal of Paint Technology in 1968, they explained, in their paper entitled "Paint Adhesion Failure Mechanisms on Steel in Corrosive Environments," that alkaline dissolution of the phosphate film is the cause of undercutting of the paint film. The alkaline environment results from the cathodic reduction of oxygen to hydroxyl ions (forming alkali-sodium hydroxide). The appropriate reactions are explained in detail in FIGS. 1 and 2 of a paper entitled "Paint Failure, Steel Surface Quality and Accelerated Corrosion Testing" by V. Hospadaruk, J. Huff, R. W. Zurilla and H. T. Greenwood, published in Society of Automotive Engineers Transactions, Section 1, Volume 87, 1978.
Simply explained, the damaged or scratched paint area begins to rust where the paint is missing within the scratch. Iron oxide is formed from the base metal by an anodic reaction in an electrolyte of water and ions of sodium chloride. The dissolution of the iron to form ferrous ions (Fe.sup.2+) is attended by the generation of electrons. Oxygen and water, which permeate the paint film in the region (which becomes cathodic) adjacent to the anodic area, then react to form hydroxyl ions. Accumulation of the hydroxyl ions results in the generation of liquid of very high basicity having a pH as high as 12.5 or more. Conventional zinc phosphate is soluble in this high basic liquid. Therefore, the paint is undercut, and a disbond between the paint film and the substrate (sheet steel) in the car body results. If the paint adjacent to the scratch is then removed, as by pulling a tape applied to the scratched area, the underlying steel surface is bright and shiny. This is due to the fact that the high pH liquid is an inhibitor for the formation of red rust. Of course, without taping, in actual use, the paint would have flaked off eventually where it has been undercut by alkaline dissolution of the phosphate film. The steel surface will then begin to rust by the mechanism common to rusting of bare steel.
D. Effect of Surface Carbon Contamination
Steel surface cleanliness, particularly contamination of automotive sheet steel by carbonaceous deposits, plays a major role in susceptibility to corrosion. The presence of even a very thin layer of carbon deposits on the steel as received from the steel mill is effective in preventing the formation of a phosphate conversion coating adequate for the best paint adhesion in the presence of an electrolyte which can support the cathodic reduction of oxygen to hydroxyl ions as described above. The reason for this is that the "pores" (either bare spots or very thin areas in the zinc phosphate conversion coating caused by the existence of carbon) act as reactive sites for initiation of the oxidation and reduction reactions. Convincing experimental evidence for this mechanism is given in the paper by R. W. Zurilla and V. Hospadaruk entitled "Quantitative Test for Zinc Phosphate Coating Quality" published in Society of Automotive Engineers Transactions, Section 1, Volume 87, 1978. This work was a breakthrough in the understanding of the parameters that control the quality of zinc phosphate coatings as a substrate for paint. This paper established that zinc phosphate coatings are porous and that a porosity of at least 1.0-1.5% is consistently encountered with substrates that have high surface carbon contamination. It is difficult to modify rolling mill practices to eliminate such surface carbon contamination, and thus deleterious porosity in phosphate films must be overcome. The porosity in such phosphate coatings is deleterious because it provides increased cathodicsites which support the electrochemical corrosion activity of the substrate. The phosphate film is then subjected to dissolution by the alkali (NaOH) generated.
E. Attempts to Reduce Corrosion Sensitivity
The main attempts by the prior art to reduce the corrosion sensitivity of phosphated metals have included (a) the introduction of inhibitors into the paint applied over the phosphate coatings to protect against corrosion, (b) the use of an inhibitor rinse such as chromic acid, which has been only partially successful in reducing the corrosion sensitivity, and (c) the use of finer phosphate crystal structure to provide a more uniform coating. These approaches have not significantly reduced the alkaline sensitivity of phosphate films.
F. Concepts Overlooked By The Prior Art
The prior art did not discover that the corrosion inhibiting property of a phosphating treatment employing zinc cations and other metal cations is critically dependent upon the relative concentrations of such cations in the bath, upon the relative concentrations of such cations in the deposited film, or upon the minimum concentration of zinc cations in a deposited coating containing cations of two or more metals.